Out of Gen 4, the only OST I really remember are the Cynthia Battle and the Giratina world music. Though I liked it, but the type of sounds it uses are way too childish, even though I understand that Pokemon IS a child game, I felt that Gen 4 music was waay too child music.
Best OST is Gen 2. The Rival theme, Rocket Team Grunt, Victory Road, Gym/Elite Four and fucking RED.
Gen 3 and 5 are also amazing. Specially 5, it has a more "mature" feel, but still beign Pokemon.
Also, I love the trumpets of Gen 3. I felt that it gave the game a "mexican" feel, which for me is great, since Hoenn is an island with the most water areas it made me think of Tenochtitlan, the city that is above a lake and all. The Pokemon designs also felt connected to it for me, with the kind of Lotad and the evolved form, or Rayquaza.